Atari STE - Issues with power connector (J302)

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HigashiJun
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Atari STE - Issues with power connector (J302)

Post by HigashiJun » Fri Jun 19, 2020 9:15 am

Could somebody help me with this weird problem happening to the onboard FDD power connector in my Atari 1040STE ?

The story so far:

As I did not like the way the floppy +5v/12v line was attached to the motherboard, I soldered a FDD 4 pin connector.

My soldering was neat but when I powered up my Atari, the +12V pin set on fire:

1.jpg
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I thought the connector had a problem, so I tried to solder a row of 4 pin header to see what would happen and surprise, no short anymore... Everything was OK and the Floppy Drive was running fine.

Then I tried to solder a new connector again and the short came back, so I soldered back the 4 pin header:

2.jpg
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But, as the PCB was burnt during these 2 shorts and the +12v through hole was a bit damaged, I decided recently to repair it with a PCB eyelet and some solder mask:

3.jpeg
3.jpeg (158.28 KiB) Viewed 1478 times

Result: the bloody short is back on the +12v pin (a small electric arc is visible at the place marked in red on the picture when i power-up the machine) and it really bothers me and makes me crazy... :headbang: The STE does not boot and I cannot go further to install all the goodies I have bought from Exxos store.

Is there a way to check the circuit with a multimeter and find where does this problem come from ?

I understand how the main power connector (J100) is connected, but I don't get the way the +12V of the FDD one is supposed to work...

(The botton of the motherboard is insulated, so it is not a problem with a pin in contact with the RF shield).

All your suggestions and ideas are welcome.

Cheers !

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Smonson
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Re: Atari STE - Issues with power connector (J302)

Post by Smonson » Fri Jun 19, 2020 10:19 am

How did you repair the PCB? You drilled out the hole and inserted a new metal tube (the eyelet?) into the hole? I've never heard of anyone doing this before.

If that's the case you may be shorting the internal ground and power layers in the PCB sandwich. I may have totally misinterpreted what is going on here, if so I apologise.

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derkom
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Re: Atari STE - Issues with power connector (J302)

Post by derkom » Fri Jun 19, 2020 10:34 am

Smonson wrote:
Fri Jun 19, 2020 10:19 am
If that's the case you may be shorting the internal ground and power layers in the PCB sandwich. I may have totally misinterpreted what is going on here, if so I apologise.
STE is just a two layer board, though, isn't it? Still could be shorting of course, but at least it shouldn't be internal.

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Re: Atari STE - Issues with power connector (J302)

Post by exxos » Fri Jun 19, 2020 10:40 am

12v isn't even used on modern floppy drives..so I don't know how you even got it to arc as there's nothing on the 12v rail.. Even so I never heard a Atari PSU arcing., it doesn't really have enough current for hat. Even if it did, its a faulty condition and the PSU would shut down.
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Re: Atari STE - Issues with power connector (J302)

Post by DoG » Fri Jun 19, 2020 11:55 am

I don't have a STe board to look at right now but isn't the ground plane surrounding the pins? It is on the STfm at least. Could it be that the solder mask is missing on the ground and made a small bridge when soldering?

It shouldn't jump/arc if enough distance is between them.

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HigashiJun
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Re: Atari STE - Issues with power connector (J302)

Post by HigashiJun » Fri Jun 19, 2020 11:57 am

exxos wrote:
Fri Jun 19, 2020 10:40 am
12v isn't even used on modern floppy drives..so I don't know how you even got it to arc as there's nothing on the 12v rail.. Even so I never heard a Atari PSU arcing., it doesn't really have enough current for hat. Even if it did, its a faulty condition and the PSU would shut down.
Yes, it surprised me too... Maybe "arc" is not the appropriated word here, but a very little spark is bouncing on the PCB at the red mark on the picture I have sent (not on the PSU, and the drive is not connected !).

I am not a beginner in repairing PCB's (lifted pads, broken traces, destroyed vias, etc.) but this kind of issue never happened to me before...

By the way, here is an example of via repair with eyelets (for Smonson):

https://www.youtube.com/watch?v=jg9fDm-uUlg

I also agree that the short is not internal, but I would like to know where is the +12v pin connected to.

Thanks to all of you.

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Re: Atari STE - Issues with power connector (J302)

Post by rubber_jonnie » Fri Jun 19, 2020 11:58 am

I think the problem you now have is that the pcb eyelet is too big and too close to the ground plane on top of the mainboard.

I'd suggest you remove it, because since the mainboard is only double sided, and the 12v rail is picked off the bottom of the board, a via is not strictly necessary, and there will be enough mechanical strength from the remaining GND and 5v pins for the socket to work.

That should effectively airgap the 12v rail from the ground plane.

Alternatively just fit the connector with 3 pins, that should be fine as you shouldn't need 12v for the floppy.

I'd suggest that the original connector is fine, just a little crispy. It is highly unlikely that it would have failed, it's just plastic with metal pins.

What is more likely is that the original via was in bad shape, and de-soldering the wires, then inserting the pins caused it to contact the ground plane which it's very close to.
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Re: Atari STE - Issues with power connector (J302)

Post by exxos » Fri Jun 19, 2020 12:06 pm

HigashiJun wrote:
Fri Jun 19, 2020 11:57 am
Yes, it surprised me too... Maybe "arc" is not the appropriated word here, but a very little spark is bouncing on the PCB at the red mark on the picture I have sent (not on the PSU, and the drive is not connected !).
Normally "arcs" like that are when you have a bad socket or pins.. once you get a bad connection it starts to burn and the problem gets worse. Using new connectors is only way. But even so, I really would just solder the cable to the MB like it was originally as your just causing your self unnecessary issues.
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HigashiJun
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Re: Atari STE - Issues with power connector (J302)

Post by HigashiJun » Fri Jun 19, 2020 12:16 pm

rubber_jonnie wrote:
Fri Jun 19, 2020 11:58 am
I think the problem you now have is that the pcb eyelet is too big and too close to the ground plane on top of the mainboard.

I'd suggest you remove it, because since the mainboard is only double sided, and the 12v rail is picked off the bottom of the board, a via is not strictly necessary, and there will be enough mechanical strength from the remaining GND and 5v pins for the socket to work.

That should effectively airgap the 12v rail from the ground plane.

Alternatively just fit the connector with 3 pins, that should be fine as you shouldn't need 12v for the floppy.

I'd suggest that the original connector is fine, just a little crispy. It is highly unlikely that it would have failed, it's just plastic with metal pins.

What is more likely is that the original via was in bad shape, and de-soldering the wires, then inserting the pins caused it to contact the ground plane which it's very close to.
Thank you rubber_jonnie and DoG for your feedback ! This grounding concern was also on my "culprits" list... I have now two same second opinions and this comforts me a lot. I will take the eyelet off and see what happens with only 3 pins connected.

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HigashiJun
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Re: Atari STE - Issues with power connector (J302)

Post by HigashiJun » Fri Jun 19, 2020 12:19 pm

Thank you Exxos.

I know simple is best, but I like complicated things :D

Your stuff is really nice but you always want to improve it too.

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